1. Field of the Invention
The present invention relates to a noise suppression apparatus, which suppresses noise mixed in an audio signal, and a control method thereof.
2. Description of the Related Art
Video cameras and recent digital cameras can capture moving images, and chances of simultaneous recording of audios are increasing. In a moving image capturing operation, wind noise mixed upon audio recording poses a serious problem, and many video cameras include a function of suppressing wind noise.
Wind noise is generated when wind strikes a microphone, and has strong components over a broad low-frequency range. On the other hand, an audio signal such as a human voice has a harmonic structure including a fundamental tone and harmonic components (components having frequencies as integer multiples of the fundamental tone).
As a conventional wind noise suppression method, high-pass filtering, spectral subtraction, comb filtering, and the like are known.
The high-pass filtering is a method of cutting strong low-frequency components of wind noise by band limitations. As a cutoff frequency determination method, a method of switching cutoff frequencies by estimating an amount of wind noise has been proposed (for example, see Japanese Patent Laid-Open No. 06-269084).
The spectral subtraction is a method of suppressing noise components by estimating wind noise included in an audio, and subtracting a spectrum of estimated noise components from that of a microphone signal (for example, Japanese Patent Laid-Open No. 2006-47639).
The comb filtering is a method which focuses attention on a harmonic structure of an audio, that is, a method of executing fundamental tone detection, and passing or cutting off a fundamental frequency and harmonic components. This method is also called a comb filter since sharp peaks or dips appear at given intervals in frequency characteristics. Noise suppression based on the comb filtering includes a method of suppressing a noise frequency band by passing a fundamental tone and harmonic components, and a method of subtracting a signal, which is obtained by cutting off a fundamental tone and harmonic components, from an original signal.
However, the conventional wind noise suppression method using the high-pass filtering, when wind noise is to be sufficiently suppressed, low-frequency components such as a fundamental tone and low-order harmonic components of an audio signal are also suppressed, and the tone color of an audio is unwantedly changed.
The method using the spectral subtraction requires noise estimation, and noise estimation accuracy has to be enhanced to obtain a satisfactory spectral subtraction result. However, since wind noise is non-stationary noise, it is difficult to attain accurate noise estimation, and noise components are unwantedly left unsuppressed due to poor noise estimation accuracy. Since wind noise includes especially strong low-frequency components, it cannot be sufficiently suppressed.
Furthermore, the method using the comb filter requires fundamental tone detection (pitch detection). Comb frequencies of the comb filter have an integer multiple relationship with respect to the fundamental frequency. For this reason, when a detected fundamental tone includes an error, an error is enlarged in a high-frequency range. The relationship between the fundamental frequency and comb frequencies is given by:fn=(f0+δ)*n where fn is an n-th comb frequency, f0 is a fundamental frequency, and δ is an error.
A fundamental tone error does not pose any problem when n is small. However, in harmonic components in a high-frequency range in which n is large, that error is enlarged in proportion to n. For this reason, an original harmonic structure may be suppressed. Since the fundamental tone detection accuracy lowers as noise is larger, accurate comb filter design suffers a problem in its feasibility.